#include <iostream>
#include <cmath>
#include <vector>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> x(n);
    vector<int> y(n);
    for (int i = 0; i < n; i++) {
        cin >> x[i] >> y[i];
    }
    int sum = 0;
    for (int i = 0; i < n; i++) {
        sum += x[i] * y[(i + 1) % n] - x[(i + 1) % n] * y[i];
    }
    cout << abs(sum) / 2 << endl;

}